With the rapid expansion of information technology industry in recent years, various information processing systems have been developed, and recording methods and recording apparatus suitable for these information processing systems have also been developed and put to practical use.
Of these recording methods, an ink jet recording method has been widely used not only in offices but also in homes from the advantages that recording is possible on various kinds of materials, hardware (apparatus) is relatively inexpensive, compact, and excellent in quietness.
With the achievement of high resolution of the ink jet printer in recent years, it becomes possible to obtain printed matters of so-called photograph-like high image quality, and with the advancement of hardware (apparatus), various kinds of recording sheets for ink jet recording have been developed.
The characteristics required of the recording sheet for ink jet recording generally include that (1) a drying speed is quick (the ink absorption speed is high), (2) the diameter of an ink dot is appropriate and uniform (free of blotting), (3) graininess is good, (4) the roundness of a dot is high, (5) color density is high, (6) saturation is high (free of lackluster), (7) the waterproofness, light fastness and ozone resistance of a print part are good, (8) the brightness of a recording sheet is high, (9) the storage stability of a recording sheet is good (yellowing discoloration is not brought about even in long term preservation, an image is not blurred under long term preservation (blurring due to aging hardly occurs)), (10) a sheet is difficult to be deformed and dimensional stability is good (curling and dimensional change are sufficiently small), and (11) the traveling performances of hardware are good (e.g., smooth feeding of sheets).
Further, in the use of photo-glossy paper used for obtaining a printed matter of so-called photograph-like high image quality, the recording sheet is also required to have glossiness, a surface smoothness, and a photographic paper-like touch analogous to that of a silver salt photograph in addition to the above various characteristics.
Sheets for ink jet recording using a coloring material-receiving layer having a porous structure are developed and put to practical use in recent years with the intention of the improvement of the above various characteristics. Such ink jet recording sheets are excellent in ink receptivity (a quick drying property) and have high glossiness due to the porous structure.
For example, JP-A-10-119423 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and JP-A-10-217601 suggest a sheet for ink jet recording comprising a support having provided thereon a coloring material-receiving layer containing fine inorganic pigment particles and a water-soluble resin and having a high porosity.
These sheets for recording, in particular, ink jet recording sheets provided with a coloring material-receiving layer comprising a porous structure in which silica is used as the inorganic pigment fine particles are excellent in ink absorption due to their structure, have ink receptivity capable of forming an image of high resolution, and have high glossiness.
However, a coloring material-receiving layer having a porous structure has two problems. In relation to the fact that a trace amount of gas in the air, in particular ozone, causes the discoloration of a printed image with the lapse of time, one problem is that a printed image is susceptible to discoloration by ozone gas in the air, since a coloring material-receiving layer having a porous structure has many pores. Accordingly, recording materials having a coloring material-receiving layer of porous structure are required to have resistance against ozone in the air (ozone resistance). Another problem is that blotting of ink is great by printing, since a coloring material-receiving layer having a porous structure has many pores. It is strongly demanded to solve the former problem mainly by the improvement of inks from the viewpoint of the development of fast coloring materials, and to solve the latter problem by the improvement of recording sheets.
In addition to the above, since many kinds of inks contain water-soluble dyes from various necessary conditions, such as operation characteristics, the defect that a printed image is liable to blot under high humidity conditions is also promoted from this aspect.
As ink jet recording methods, a method of pressurizing ink by a piezoid and discharging ink droplets, a method of generating bubbles in ink by heat and discharging ink droplets, a method of using ultrasonic waves, and a method of sucking and discharging ink droplets by electrostatic force are known. As the ink compositions for these ink jet recording methods, water-based inks, oil-based inks and solid inks (a melting type) are used. Of these inks, water-based inks are mainly used from the points of handling properties, odor and safety.
Colorants for use in the inks for these ink jet recording methods are required to be high in solubility in a solvent, capable of high density recording, good in hue, fast to light, heat, air, water and chemicals, good in fixing ability to image-receiving materials and not liable to blot, excellent in storage stability as inks, nonpoisonous, high in purity, and available inexpensively. However, it is extremely difficult to find out colorants which satisfy these requirements on high levels. Various kinds of dyes and pigments have been suggested and brought into practical use for ink jet recording, however, a colorant which satisfies all of these requirements is not found yet under the present conditions. In conventionally well-known dyes and pigments attached with color index Nos. (C.I. Nos.), those which sufficiently satisfy necessary characteristics required of the ink for ink jet recording including hue and fastness have not been found yet. The development of dyes and pigments having good hue and fastness as the colorants for ink jet recording has been advanced aggressively, but, additionally, the solution stability and dispersion stability of ink are also important characteristics concerning the stable operation of apparatus and the storage stability of ink.
With respect to this point, although water-soluble dyes are substituted with water-soluble groups and these water-soluble groups are excellent in the point of their contribution to the stability of water-soluble dyes in water-based ink, water-soluble groups also have a weak point that they are liable to cause blotting of recorded images, and images formed are liable to blot under high humidity conditions when the number of water-soluble groups of dyes and pigments is increased for improving the stability of ink. Therefore, there is a problem of incompatibility of the stability in ink with blotting resistance.